(Invited) Benchmarking Carbon-Supported Pt-Based Oxygen Reduction Reaction Electrocatalysts for PEMFC Cathodes

Abstract

The present performance of state-of-the-art proton exchange membrane fuel cells (PEMFC) enable their commercialization, as demonstrated by the recent release of fuel-cell powered vehicles (forklifts by Plug Power, FCEV Miraï by Toyota, etc.). Now, all PEMFC manufacturers need to reduce the PEMFC systems initial cost and to enhance their durability and reliability in operation to reach the market requirements and industrial sustainability. To this goal, one must develop highly-efficient oxygen reduction reaction (ORR) electrocatalysts, which was claimed to be obtained by numerous groups in a recent past [1-6]. Unfortunately, the scientific community still struggles to obtain the best performance of these materials in operating PEFMCs: their practical performance in membrane electrode assembly (MEA) do not approach those expected from an analytical approach performed using the rotating disk electrode setup, RDE (the most widely-used setup to assess a material’s ORR performance).There are several reasons that may explain why RDE data are not relevant to account for MEA data. Firstly, in RDE, the O2 reactant is fed as gas dissolved in the electrolyte (slow diffusivity and solubility), hence the mass-transfer rate and related limiting current density are ca. 3 orders of magnitude lower than in real PEMFC conditions. Secondly, the activity data is only extracted at high potential values in RDE (E > 0.85 V vs RHE), although PEMFC optimal performance is reached in the 0.6 – 0.8 V vs RHE cathode potential range. Thirdly, the proton conduction can be limiting in the thin layer of ionomer covering the electrocatalyst nanoparticles in MEA, which could limit their operation, especially at the high current densities at stake in MEA. In essence, one is not sure that the electrocatalysts are used in optimized conditions in MEA, as emphasized mass-transport of reactants (both H+ and O2) to the catalytic sites is not granted for present ionomers and MEA structure [7, 8]. Recent results obtained in newly-developed electrochemical characterization setups like the floating electrode (FE) [9], the gas-diffusion electrode (GDE) [10] or differential cell (DC), could enable to reach much higher ORR current densities at the lab scale and therefore better match MEA data, a prerequisite for relevant benchmarking.In the present contribution, these setups will be used to characterize three state-of-the-art ORR electrocatalysts (50 wt% Pt/Vulcan XC72, 50 wt% Pt3Co/Vulcan XC72 and 30 wt% Pt/graphitized carbon black), and comparison will be made with large single-cell PEMFC data in automotive conditions. We will show whether the “RDE promises” of alloyed electrocatalysts (measured in RDE configuration) do maintain in FE, GDE and DC, and whether the “advanced electrocatalysts” still exhibit their plain potential in real PEMFC catalytic layer configuration.